1. Field of the Invention
The invention relates to a tunable receiver input circuit comprising an amplifying three-pole circuit, a tunable resonant network having the characteristic of a series resonant circuit, the three-pole circuit being negatively fedback by means of the tunable resonant network, and a mixing circuit coupled to the three-pole circuit.
2. Description of The Prior Art
Such a tunable receiver input circuit is disclosed in German Patent Application No. 29 52 793, which has been laid open to public inspection and is inter alia used in car radios having a passive or a so-called active antenna.
The tunable receiver input circuit with mixing stage has for its object to select an antenna signal and to convert this signal into an intermediate-frequency signal of the receiver by means of an oscillator. Such a conversion must be as low in noise as possible and must be effected as free from interferences caused by non-linear effects as possible.
The said German Application No. 29 52 793, which has been laid open to public inspection discloses a tunable receiver input circuit having a receiving section connected across a portion of the resonant network, this receiver section comprising a mixing circuit. Such a circuit has the disadvantage that the mixing circuit is driven without further linearisation measures being taken. This results in second and third order noise products being generated due to non-linearity of the mixing circuit itself, in spite of the fact that the received signals applied to the mixing circuit are preselected by the resonant network and consequently the mixing circuit is adequately protected from interferences at the mirror signal frequency.
In receiving areas in which the available frequency bands are very crowded this results, more specifically in the case of very powerful transmitters, in a highly disturbed reception by combined second and third order frequencies.